The present invention generally relates to imaging and image-guided navigation. In particular, the present invention relates to a system and method for improved image-guided navigation by distortion mapping using integral methods.
Medical practitioners, such as doctors, surgeons, and other medical professionals, often rely upon technology when performing a medical procedure, such as image-guided surgery or examination. A tracking system may provide positioning information for the medical instrument with respect to the patient or a reference coordinate system, for example. A medical practitioner may refer to the tracking system to ascertain the position of the medical instrument when the instrument is not within the practitioner's line of sight. A tracking system may also aid in pre-surgical planning.
The tracking or navigation system allows the medical practitioner to visualize the patient's anatomy and track the position and orientation of the instrument. The medical practitioner may use the tracking system to determine when the instrument is positioned in a desired location. The medical practitioner may locate and operate on a desired or injured area while avoiding other structures. Increased precision in locating medical instruments within a patient may provide for a less invasive medical procedure by facilitating improved control over smaller instruments having less impact on the patient. Improved control and precision with smaller, more refined instruments may also reduce risks associated with more invasive procedures such as open surgery.
Tracking systems may also be used to track the position of items other than medical instruments in a variety of applications. That is, a tracking system may be used in other settings where the position of an instrument in an object or an environment is difficult to accurately determine by visual inspection. For example, tracking technology may be used in forensic or security applications. Retail stores may use tracking technology to prevent theft of merchandise. In such cases, a passive transponder may be located on the merchandise. A transmitter may be strategically located within the retail facility. The transmitter emits an excitation signal at a frequency that is designed to produce a response from a transponder. When merchandise carrying a transponder is located within the transmission range of the transmitter, the transponder produces a response signal that is detected by a receiver. The receiver then determines the location of the transponder based upon characteristics of the response signal.
Tracking systems are also often used in virtual reality systems or simulators. Tracking systems may be used to monitor the position of a person in a simulated environment. A transponder or transponders may be located on a person or object. A transmitter emits an excitation signal and a transponder produces a response signal. The response signal is detected by a receiver. The signal emitted by the transponder may then be used to monitor the position of a person or object in a simulated environment.
Tracking systems may be ultrasound, inertial position, or electromagnetic tracking systems, for example. Electromagnetic tracking systems may employ coils as receivers and transmitters. Typically, an electromagnetic tracking system is configured in an industry-standard coil architecture (ISCA). ISCA uses three colocated orthogonal quasi-dipole transmitter coils and three colocated quasi-dipole receiver coils. Other systems may use three large, non-dipole, non-colocated transmitter coils with three colocated quasi-dipole receiver coils. Another tracking system architecture uses an array of six or more transmitter coils spread out in space and one or more quasi-dipole receiver coils. Alternatively, a single quasi-dipole transmitter coil may be used with an array of six or more receivers spread out in space.
The ISCA tracker architecture uses a three-axis dipole coil transmitter and a three-axis dipole coil receiver. Each three-axis transmitter or receiver is built so that the three coils exhibit the same effective area, are oriented orthogonally to one another, and are centered at the same point. If the coils are small enough compared to a distance between the transmitter and receiver, then the coil may exhibit dipole behavior. Magnetic fields generated by the trio of transmitter coils may be detected by the trio of receiver coils. Using three approximately concentrically positioned transmitter coils and three approximately concentrically positioned receiver coils, for example, nine parameter measurements may be obtained. From the nine parameter measurements and one known position or orientation parameter, a position and orientation calculation may determine position and orientation information for each of the transmitter coils with respect to the receiver coil trio with three degrees of freedom.
In medical and surgical imaging, such as intraoperative or perioperative imaging, images are formed of a region of a patient's body. The images are used to aid in an ongoing procedure with a surgical tool or instrument applied to the patient and tracked in relation to a reference coordinate system formed from the images. Image-guided surgery is of a special utility in surgical procedures such as brain surgery and arthroscopic procedures on the knee, wrist, shoulder or spine, as well as certain types of angiography, cardiac procedures, interventional radiology and biopsies in which x-ray images may be taken to display, correct the position of, or otherwise navigate a tool or instrument involved in the procedure.
Several areas of surgery involve very precise planning and control for placement of an elongated probe or other article in tissue or bone that is internal or difficult to view directly. In particular, for brain surgery, stereotactic frames that define an entry point, probe angle and probe depth are used to access a site in the brain, generally in conjunction with previously compiled three-dimensional diagnostic images, such as MRI, PET or CT scan images, which provide accurate tissue images. For placement of pedicle screws in the spine, where visual and fluoroscopic imaging directions may not capture an axial view to center a profile of an insertion path in bone, such systems have also been useful.
Generally, image-guided surgery systems operate with an image display which is positioned in a surgeon's field of view and which displays a few panels such as a selected MRI image and several x-ray or fluoroscopic views taken from different angles. Three-dimensional diagnostic images typically have a spatial resolution that is both rectilinear and accurate to within a very small tolerance, such as to within one millimeter or less. By contrast, fluoroscopic views may be distorted. The fluoroscopic views are shadowgraphic in that they represent the density of all tissue through which the conical x-ray beam has passed. In tool navigation systems, the display visible to the surgeon may show an image of a surgical tool, biopsy instrument, pedicle screw, probe or other device projected onto a fluoroscopic image, so that the surgeon may visualize the orientation of the surgical instrument in relation to the imaged patient anatomy. An appropriate reconstructed CT or MRI image, which may correspond to the tracked coordinates of the probe tip, may also be displayed.
Medical practitioners, for example, rely on electromagnetic trackers to perform sensitive image-guided surgery. Accuracy of position measurement is important when guiding a precision instrument in a patient without a direct line of sight. Distortion may produce inaccurate position measurements and potential danger to a patient. Thus, a system that reduces inaccurate tracking measurements would be highly desirable. A system that minimizes the effect of distortion on position measurement would be highly desirable.
Distortion may be introduced into an image from magnetic fields generated by tools, sensors, and the examination or operating environment, for example. Current systems typically correct for distortion by mapping position and orientation of the distorting object. Position mapping uses a model that ignores the distortion and corrects for the distortion of the position and orientation of the distorting object are modeled. However, a model and correction scheme that ignore the effects of distortion in the model are inefficient and inaccurate. Thus, a system and method for improved distortion modeling and correction would be highly desirable.
Additionally, when mapping position and orientation of a distorter, a differential method is used. That is, the magnetic field is measured on the surface of the distorter. Then, a series of differential calculations are computed to determine a table of values representing the interior of the volume. That is, a field is measured at the surface of the volume and differential equations are directly solved for the interior of the volume to construct a look-up table model. Such a look-up table determination involves a large number of calculations. Thus, a simplified method for determining field distortion of a volume would be highly desirable.
Thus, there is a need for an image-guided navigation system and method for improved distortion mapping using integral methods.